I think it depends on sight lines. With tens of thousands of miles under my belt, and as a committed vehicular cyclist, there are roads I don't ride.

You say the road is flat ... is it also straight? If so, and if there's really no alternative, I would go ahead and ride it, being sure to take the lane. If it's curvy so that traffic behind you might not see you well in advance, and oncoming traffic might not see a car in their lane trying to pass you, I'd just stay off it.